1. Technical Field
The present invention relates to an exposure head including a plurality of light emitting elements, an image forming unit having the exposure head, and an image forming apparatus having the exposure head.
2. Related Art
As one of exposure heads that are used in image forming apparatuses that form an image by transferring a latent image formed on the surface of a photosensitive drum by exposing the photosensitive drum to a medium such as a paper sheet through a primary transfer roller or the like, there is a type of an exposure head that uses light emitting elements regularly disposed on a substrate and gradient index lenses. In the above-described type of the exposure head, a plurality of the gradient index lenses forms images to be superposed in a same position on the surface of a photosensitive drum by using light emitted from one light emitting element, and thereby one spot is formed on the surface. By aggregating the spots, an image (latent image) is formed.
As a problem that can occur in the above-described type of the exposure head, there is a deviation of amounts of light among the plurality of light emitting elements. When a same current or voltage is applied, the intensity of light emitted from each light emitting element changes by time due to the deviation of the occurrence frequency of light emission so as to generate a deviation, and accordingly, formation of an excellent image (latent image) is hindered. In order to suppress the above-described phenomenon, for example, in an exposure head disclosed in JP-A-2004-66758, the amount of light for each light emitting element can be measured by disposing light detecting units together with the light emitting elements on a light emitting substrate.
FIGS. 16A, 16B, and 16C schematically show a light emitting substrate 300 that uses bottom-emission type organic EL elements 310 as the light emitting elements, as a light emitting substrate included in the above-described type of the exposure head, that is, a general exposure head. FIG. 16A is a top view of the light emitting substrate 300, FIG. 16B is a side view thereof, and FIG. 16C is a cross-section view taken along line XVIC-XVIC. As shown in the figures, on the periphery of the organic EL elements 310 disposed on a transparent substrate 301 in a zigzag pattern, light detecting units 320 that are formed of light receiving elements such as photo diodes as units for detecting light are disposed.
It is configured that a part of light emitted from the organic EL element 310 is repeatedly reflected between the front and rear surfaces of the transparent substrate 301 so as to reach the light detecting units 320. The amounts of light can be measured by sequentially turning on the organic EL elements 310. Then, by correcting values of the currents (or the voltages) applied to the organic EL elements 310 based on the result of measurement, an image (latent image) having high definition can be formed.
However, the above-described light emitting substrate has a problem that the ratio of light, which can be measured by the light detecting unit 320, to light emitted from the organic EL element 310 is low. In other words, as shown in FIG. 16C, the light reaching the light detecting unit 320 is limited to light L1 within the angle range θa. Thus, light beyond the angle range is emitted to the outside of the light emitting substrate 300 without reaching the light detecting unit 320. Accordingly, it is difficult to accurately acquire the aging change in the intensity of light emission of the organic EL element 310. In addition, there is a problem that accuracy of the above-described correction may decrease in a case where a complex amplification circuit, a noise reduction circuit, or the like is not used.